Используйте оператор RxJs 'tap
: stackblitz
ngOnInit() {
this.filteredOptions = this.myControl.valueChanges
.pipe(
startWith<string | User>(''),
map(value => typeof value === 'string' ? value : value.name),
map(name => name ? this._filter(name) : this.options.slice()),
tap(() => this.myControl.setValue(this.options[0]))
);
}